Although I certainly wish you a warm welcome to the CameraLabs forum I'll have to request that you take a moment to read our forum rules and, in particular, the section entitled Got a question?. I've deleted your other three identical posts.

A55 is more video camera then photo. I bought A580 recently, and I totally love it. High iso performance is incredible, live view works perfectly. I was thinking about D5100, but it's not as good as A580. The best thing to do is go to store and touch them, take some photos and see how it works for you.

The D5100 has perhaps the best low light performance but it is supposedly very noisy at the top end as you'd expect.

The lens you're using will make the greatest difference to low light performance between the three - a fast lens will allow you to shoot at a lower ISO, thereby reducing the amount of image noise.

I can't say I agree much with hunker7's comment about the a55 being more of a video camera than a camera. It uses a transparent mirror so it's perhaps not a DSLR by definition but it's definitely more aimed at photography than video.

With the 550D and D5100, neither body has image stabilisation - that is entirely dependent on the lens you're attaching. You should also bear in mind that neither Canon nor Nikon's 50mm f/1.8 lenses have image stabilisation though given their wide aperture, you can shoot at a higher shutter speed anyway.

As suggested, you're best off trying all of the said cameras out and deciding which you feel most comfortable using.

As others mentioned, feel in your hands means a lot - because you're not going to shoot as much with a camera you don't enjoy holding and shooting with.

However, based purely on the specs you listed as important to you, the Sony SLT and DSLR cameras clearly have the best match. When it comes to live view full-time, Sony is clearly leaps and bounds over any other DSLR - they are the only cameras that realistically can be used in live view for any type of shooting, the only ones that have zero shutter delay or mirror lag, can use full time phase detection autofocus while in live view, and even shoot up to 7 frames or more per second in live view. For low light, the A55 & A580 share the same excellent sensor (the same also in the Nikon D5100 & 7000), with the A580 getting a slight advantage in the very highest ISOs due to the slight light lost to the A55's translucent mirror. And the body-based stabilization systems will give a nice advantage in low light use in being able to use fast standard primes in low light and have them stabilized...something you can't get in lens-based stabilization.

Do check them all out - until you've actually laid hands-on, and tried them out, you may find some features more important than you thought, or not as important as you thought. And you definitely want to be comfortable and happy holding and using the camera. Best of luck.

probably get slated by somepeople here . i would never buy a sony dslr just because theres no really good pro version so if you ever decided to take that step in the future you would have to sell everything and start again. i could be wrong though, from what i hear sony cameras are getting pretty good now days

Well while everyone's entitled to their own opinions and reasonings, I personally don't go for the 'pro' argument because I personally have no desire to buy a 'pro' body camera, or to attempt to make my entire living from photography. The word 'professional' is loose when it comes to photography - what precisely makes a person a 'pro' and how many people who own cameras are 'pros' versus how many are not? I sell photos, I have been published in national magazines, and have been hired to shoot events. Am I a pro? All of my camera equipment has been paid for by photography. Am I a pro? My 'career' isn't photography - I earn my living doing something else, but photography is fun, and I've grown in it enough that it has become a self-funding hobby...but I wouldn't consider myself a 'pro'. I've managed to do everything I've needed to do with Sony cameras, mostly because I pick the tools that will work best for my needs, and put very simply, no other camera can do what I want to do with them - only Sony. So that's the direction I went. Support? What support do I need? I want to buy a new lens, I have more than 110 to choose from. I want third party lenses, I've got 3 major brands making lenses for my mount. I want used lenses, I've got a huge backlog of excellent Minolta lenses available. I haven't had need of repair or abnormal levels of customer service, because I'm not working for National Geogreaphic and shooting at the Tsavo River with a failed 600mm lens that needs air-shipped replacement with on-site delivery by next day to meet my publication deadline.

Let's put it this way - do you buy every product in your life based on whether it's the one 'professionals' use? Do you drive only Mercedes or Ferrari, because the winners of Formula One drive those? Cook only on Viking ranges because that's what pro chefs use? Or do you buy a good product that does what you need it to do, hopefully with good features, good style, and a good price?

If you are indeed a professional or aspire to be one, of the very highest definitions of professional photography - full-time, career photographer depending on your camera to feed the children...by all means shop Canon and Nikon and forget the rest - they have a very particular level of support that a professional photographer in the field needs when their living depends entirely on their cameras and lenses. But don't forget to only buy Canon or Nikon lenses too - because third party lenses aren't going to give you that same level of support. Remember too that you'll be spending tens of thousands of dollars for the pro level bodies and lenses required to receive that support.

For any other type of photographer, from the family P&Ser to the semi-pro photog who earns some side-income off their photography, pick the best tool to accomplish your job, and enjoy the wider selection of fine cameras available to you.

Not that it matters what some uninformed folks think, but there is lots of "pro" equipment in the lineup. I've had my a700 in for some minor repair twice and each time I'm given another a700 as a loner while waiting (no charge).
Working for a national media company all our broadcast equipment is Sony, and now there is talk that they will probably want us to go with the new a77 for it's video capabilities as a backup to stills if needed.